a) Field of the Invention
The present invention relates to a liquid crystal display and a method of driving the same, and more particularly relates to a field-control nematic liquid crystal display including a liquid crystal cell disposed between a pair of polarizers and a method of driving the same.
b) Description of the Related Art
Heretofore, the following two types of electrooptic effect devices are known in the case of observing a nematic liquid crystal cell disposed between a pair of polarizers under electrical control.
First Type: Twisted Nematic Type PA1 Second Type: Electrically Controlled Birefringence type
In the following, the electrooptic effect devices of these two types are described with reference to FIGS. 2A and 2B.
FIG. 2A shows a conventional twisted nematic liquid crystal display. In FIG. 2A, the left part expresses an "OFF" state and the right part expresses an "ON" state. In the "OFF" state, the liquid crystal layer is in a twisted state. The twisting of the liquid crystal layer can be obtained by arranging a pair of substrates 21 and 22 to be in a twisted state and arranging liquid crystal molecules 23 adjacent to the substrates 21 and 22 to be in the non-parallel axial directions. The liquid crystal cell as shown in FIG. 2A has a twist angle of 90.degree.. Further, there is also known a so-called supertwisted nematic liquid crystal display using a larger twist angle. In FIG. 2A, the arrow shows an axial direction of dielectric anisotropy. In this case, the dielectric anisotropy of the liquid crystals is positive.
In the "ON" state, a voltage is applied between the substrates 21 and 22. The direction of liquid crystal molecules 23 at the intermediate portion of the liquid crystal layer apart from the substrates 21 and 22 is changed toward the direction of the electric field by this voltage application. In short, in the "ON" state, the liquid crystal molecules 23 are twisted and tilted. The tilt angle can be controlled electrically.
In the following, the operation of the twisted nematic type liquid crystal display is described. In the "OFF" state, the liquid crystal cell serves as a waveguide. Polarized light causes rotation of the polarization axis along with the twisted liquid crystal layer. In the case where the first and second polarizers 25 and 26 provided on opposite sides of the cell have parallel polarization axes P1 and P2 and the polarization axis P1 of the first polarizer 25 is arranged in the X-direction to be parallel to the direction of orientation of the liquid crystals, light transmitted through the first polarizer 25 enters into the liquid crystal layer and is rotated to form Y-direction polarized light. The Y-direction polarized light is entirely absorbed by the second polarizer 26 having the X-direction polarization axis P2.
In the "ON" state, the waveguide effect of the liquid crystal layer is lost by tilting the liquid crystal molecules. As the liquid crystal molecules in the cell are tilted more intensively, the transmittance of the cell increases.
As described above, the "OFF" state in which light is not transmitted and the "ON" state in which light is transmitted can be obtained.
FIG. 2B shows an electrically controlled birefringence type liquid crystal display. The liquid crystal cell is arranged between first and second polarizers 30 and 31. For example, the first polarizer 30 has a polarization axis P1 in the X-direction and the second polarizer 31 has a polarization axis P2 in the Y-direction perpendicularly crossing the X-direction. The liquid crystal molecules 29 have negative dielectric anisotropy. In FIG. 2B, the left part expresses an "OFF" state and the right part expresses an "ON" state.
In the "OFF" state, the liquid crystal molecules 29 are in a homeotropic state in which they are perpendicular to the substrates 27 and 28. When a voltage is applied between the substrates 27 and 28, the liquid crystal molecules 29 are tilted according to the negative dielectric anisotropy. Accordingly, the "ON" state exhibits a tilt state.
In the "OFF" state, the liquid crystal cell has no optical effect on incident light. Because the liquid crystal cell is arranged between the crossed polarizers, the incident light is absorbed by the crossed polarizers 30 and 31. These polarizers 30 and 31 may be constituted by linear polarizers or may be constituted by circular polarizers.
In the "ON" state, the liquid crystal cell exhibits the effect of birefringence. In the case of using crossed linear polarizers, the maximum value of transmittance can be obtained when angles of .+-.45.degree. are formed between the tilt direction of the liquid crystal molecules 29 and the polarization axes of the polarizers 30 and 31. In the case of using circular polarizers, the maximum value of transmittance can be obtained at the middle of the respective polarizer. In the case of using crossed linear polarizers, the effect of birefringence is lost when one of the polarizers is aligned with the tilt direction of the liquid crystal molecules.
In either case of the twisted nematic type and the electrically controlled birefringence type, the tilting of the liquid crystal molecules is controlled by the electric field.
In the aforementioned liquid crystal display, the impressed electric field has no relation to the twisting of the liquid crystal layer. In the case of the twisted nematic type, the twisting exists not only in the "ON" state but in the "OFF" state. In the case of the electrically controlled birefringence type, the twisting exists neither in the "OFF" state nor in the "ON" state.